Hi,
Im trying to overload a multiplication operator for scalars and vectors,
but keep running into the error message "Functional dependencies
conflict". What I think is going on is that the dependency check doesn't
work with incoherent (or overlapping) instances. In the example below,
the two instances of Mult are overlapping. What I want is the vector
version to be used for vectors and the scalar version used for numbers,
even if a vector-type is an instance of the Num-class (I believe
-fallow-incoherent-instances would make that kind of choise for me,
right?).
Im using the Visual Studio plugin Visual Haskell, and thus GHC version
6.6. Otherwise I think associated types might have worked better for
this...
Regards
Tobias Bexelius
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances
-fallow-incoherent-instances #-}
data V2 a = V2 a a
class Vec v a where
dot :: v a -> v a -> a
instance Num a => Vec V2 a where
V2 a1 a2 `dot` V2 b1 b2 = a1*b1+a2*b2
class Mult a b c | a b -> c where
(*.) :: a -> b -> c
instance (Num x) => Mult x x x where (*.) = (*)
instance (Vec a x) => Mult (a x) (a x) x where (*.) = dot